Industry increasingly desires polyimide films for electronic application to be matte in appearance, have a specific color, durability to handling and circuit processing, and when used as a coverlay, provide security against unwanted visual inspection of the electronic components protected by the coverlay. Single layer matte luster films do not have an L* color less than 30 providing deep, rich saturated colors desired by industry. Typically, as the amount of matting agent is increased the color of the film becomes muted. The effect of increased surface roughness from the matting agent is the dilution of the pigment color so that it appears lighter and less saturated. This is caused by the dilution of the diffuse reflectance (where pigment color is perceived) by the increased scatter of the specular reflectance (white light). The rougher the surface, the lower the gloss and greater the scatter of the specular reflectance. Thus, as gloss decreases, L* (lightness) typically increases. Adding more colorant does not decrease the L* color. Thus, simultaneously achieving low gloss and low L* color is therefore difficult.
For the forgoing reasons, a need exists for a polyimide film that is matte in appearance, has deep, rich saturated colors, as well as provide sufficient optical density to provide visual security when used as a coverlay while having acceptable electrical properties (e.g., dielectric strength) mechanical properties, and durability to handling and circuit processing.